"A certain group one metal combines with Hydrogen, to form a molecule with the formula XH. The equilibrium bond length of this compound is 1.5957 Å. If this molecule exhibits a pure rotational spectrum with a rotational constant of 7.5131cm−1, calculate the molecular weight of metal X."

I thought that you would use a mix of the reduced mass and rotational constant formulas to get:

[tex]1.66e-27(\frac{(m1)(m2)}{m1+m2})=\frac{h/2\pi}{4\pi Bcr^{2} }[/tex]

However, that just gives a negative answer, and mass cannot be negative. I am confused and would appreciate any help